Add conversion methods for `PresignedRequest` and update docs (#1116)

* update: s3 presigning example with curl command generator
add: impl into for PresignedRequest -> http::request::Builder
sort: cargo deps

* update: CHANGELOG.next.toml

* appease: clippy

* add: struct conversion doc links
add: fn for PresignedRequest -> http::Request<B>
update: expand presigning example

* update: changelog
move: declaration of From<PresignedRequest> for http::request::Builder

* remove: leftover import